Thermoquad idle issue

Did you say you can find no vacuum leaks? How did you check for vacuum leaks? I'm assuming the engine has been rebuilt: has the deck been cut? the heads been cut? how much? has the intake been cut? how many miles since rebuild? does the engine use oil, especially at freeway speeds? is there an oil film on the back of the car after freeway speed cruising? Was the cam dialed-in? Did you take a picture of the timing gears/chain when at TDC, before you buttoned it up? You mentioned "mild" cam, cam specs?
What I'm getting at is that the vacuum seems low. Is the vacuum steady or fluctuating? If steady, but low: either a vacuum leak above the intake, or below the intake, or late valve timing.
If fluctuating: sticking valve, bad valve spring, valve(s)not seating.